Engine manufacturers are mandated to detect engine misfire events as an element of state and federal on-board diagnostics regulations. The regulations require detection of various forms of engine misfire, including intermittent misfire, one cylinder continuous misfire, and two opposing cylinder misfire tests. Designers and manufacturers of internal combustion engines employ various detection schemes, including detection of misfire based upon variations in rotational speed of the engine crankshaft. Crankshaft speed analysis is the adopted approach for many engine applications due to its direct linkage to engine torque. Engine torque is the primary quantity affected upon occurrence of cylinder misfire, and crankshaft speed variation provides a direct means to detect associated changes.
Crankshaft speed variation monitoring algorithms to monitor engine operation may be unable to identify all occurrences of engine misfire, due to variations in system implementation that introduce errors inducing signal noise. Signal noise may affect the ability of a misfire detection method to detect misfire, especially at high engine speeds on some engine applications. Issues related to system implementation that may affect signal noise include, for example, part-to-part variation in crankshaft wheel tooth machining, and variations in engine torque between cylinders, due to variations in delivery of fuel, air, spark, and exhaust gas recirculation. Variation in signal level, i.e., change in engine speed due to occurrence of a misfire, is necessarily low under conditions of low engine load. In general, misfire detection is difficult under low load, high-speed conditions, especially for engines with a large number of cylinders and large levels of powertrain and driveline inertia. Furthermore, misdiagnosis of engine misfire events may lead to costly, and potentially ineffective service procedures.
Other methods have been proposed to improve misfire detection, including, by way of example, U.S. Pat. No. 5,668,725, entitled: Internal Combustion Engine Misfire Detection, issued to Naik, comprising a method and apparatus for monitoring engine speed to detect intermittent engine misfire events. The aforementioned crankshaft speed variation method significantly improves the misfire detection accuracy of intermittent misfires. The method may be unable to detect continuous single cylinder misfires in specific engine applications, e.g., wherein an individual cylinder continuously misfires within a specific speed/load operating range otherwise undetectable using normal monitoring methods. In such instances, a misfiring cylinder may exhibit itself as a periodic signal consistent within a reference period and having a period of one engine cycle. This periodic sequence is estimated and removed from the data, and the resultant output of the method may not be detected.
Therefore, what is needed is a more effective method to monitor engine speed variation to detect single cylinder engine misfire, especially such misfire events as may continuously occur at specific engine speed/load operating points.